With reference to FIG. 1, a conventional LCoS projection system includes a light source 6 for generating a high power beam of light, and a polarization conversion light pipe (PCLP) 7 for ensuring substantially all of the light is linearly polarized, e.g. horizontally polarized. A first lensing element 8a passes the beam of light to a first folding mirror 9a, which directs the beam of light at a first dichroic filter 10. The first dichroic filter 10 separates out green light from the remaining light, and directs the green light via second and third lensing elements 8b and 8c, and second and third folding mirrors 9b and 9c to a first LCoS display panel 15a. Prior to entering the first LCoS display panel 15a, the green light passes through a first polarizer 20a, such as a wire grid polarizer, and a first trim retarder PCE 21′. The polarization of a desired amount of the green light is rotated by the first display panel 15a, e.g. by 90° to horizontal polarization, whereby upon reflection the desired amount of green light is reflected by the first polarizer 20a towards the color cube 19 for transmission via the projection lens 18 onto a screen (not shown).
The remaining light is directed via fourth and fifth lensing elements 8d and 8e, and a fourth folding mirror 9d to a second dichroic filter 12, which separates the remaining light into blue and red light. The red light is directed through a second polarizer 20b and a second trim retarder PCE 21″ to a second LCoS display panel 15b. As above, the second display panel rotates the polarization of the desired amount of red light, whereby the polarizer 20b reflects the red light to the color cube 19 for transmission via the projection lens 18 onto the screen (not shown).
Similarly, the blue light passes through a third polarizer 20c and a third PCE 21′″ to a third LCoS display panel 15c. The polarization of a desired amount of blue light is rotated, whereby upon reflection, the blue light is reflected by the third polarizer 20c to the color cube 19 for transmission via the projection lens 18 onto the screen (not shown).
LCoS micro-displays typically have some residual birefringence in the dark (off-) state due to the orientation of the liquid crystal (LC) molecules in the LCoS cell. Thus, the linearly polarized light that illuminates the micro-display is slightly elliptically polarized upon reflection, which leads to dark state light leakage onto the screen, thereby limiting the contrast of the projection system. To improve the system contrast and compensate for the residual retardance, the trim retarder PCE 21′ is placed in the light path in front of the LCoS display panel 15a. Unlike a conventional ½λ waveplate, a trim retarder provides only about 1 nm to 50 nm of retardance. The PCE 21′ prepares the polarization state of the illuminating light, whereby upon reflection from the LCoS display panel 15a back through the PCE 21′ the polarization state of the reflected light is effectively unchanged, i.e. rotated back to its original state of polarization, and no light is leaked onto the screen. It is also important that the polarization state of light reflected off of the PCE 21′ is effectively unchanged or it will contribute to a loss in system contrast.
U.S. Pat. No. 6,215,539 issued to Schadt et al on Apr. 10, 2001, and U.S. Pat. No. 6,160,597 issued to Schadt et al on Dec. 12, 2000 teach a method of manufacturing birefringent films that effect the polarization state of light for use in conventional ¼λ and ½λ waveplates. A linearly photo-polymerizable polymer (LPP) is used to orient a photo-polymerizable liquid crystal polymer (LCP) film by coating a transparent low-birefringence substrate with a thin layer of the LPP, aligning the LPP layer by exposure to linearly polarized light, and coating the photo aligned layer of LPP with the LCP layer. During thermal treatment of the structure, the oriented LPP layer induces (nucleates) orientation in the photo-polymerizable LCP layer. The oriented photo-polymerizable LCP layer is then fixed by exposure to light of sufficient energy to cause photo-induced cross-linking thereof.
An object of the present invention is to overcome the shortcomings of the prior art by utilizing a LPP/LCP polarization film as a trim retarder PCE for an LC display panel.
Another object of the present invention is to provide a LPP/LCP PCE having a LCP layer with a low ΔN birefringence value.
Yet another object of the present invention is to reduce the amount of reflection and interference events in a LPP/LCP PCE by introducing dielectric layers at suitable material interfaces.